POS0458 ENRICHMENT OF COMPLEMENT, IMMUNOGLOBULINS, AND AUTOANTIBODY TARGETS IN THE PROTEOME OF PLATELETS FROM PATIENTS WITH SYSTEMIC LUPUS ERYTHEMATOSUS (SLE)
Authors: P. Linge, A. Jern, H. Tydén, B. Gullstrand, H. Yan, C. Welinder, R. Kahn, A. Jonsen, J. Semple, A. Bengtsson
Journal: Annals of the Rheumatic Diseases (2022)

Background Systemic Lupus Erythematosus (SLE) is characterized by autoimmunity towards apoptotic/necrotic cells, complement activation and excessive amounts of circulating immune complexes. Platelets are recognized as immune cells that interacts with innate and adaptive immune functions. They are activated in SLE patients and contribute to an increased susceptibility to thrombosis [1]. Decreased platelet size has been observed in patients with SLE [2], but the mechanism(s) remains unclear. In this study, we have analyzed the complete proteome of platelets with normal and decreased size from SLE patients and from healthy controls (HC). Objectives Our aim was to find clues that could explain the morphological differences observed in platelets from SLE patients and to better characterize the role of platelets in SLE. Methods We included 23 consecutive patients with SLE, median SLEDAI-2K score was 2, and 10 HC. Blood count, serum complement levels and the presence of antiphospholipid or dsDNA antibodies were analyzed in all patients. Platelet size (forward scatter) and activation status (CD154, PAC1, CD32, PAR1, CD62P and Annexin V) was determined using flow cytometry. The proteome of 10 platelet isolates from SLE (five with smallest and the five with largest average size) and five HC were analyzed using liquid chromatography with tandem mass spectrometry (LC-MS/MS). Data were analyzed using ANOVA, t-test, hierarchical cluster analysis, protein interactions using the STRING software and correlation analysis using spearman correlation. Results We identified a total of 2572 proteins from the platelet isolates. Out of the identified proteins, 396 had significantly different levels, meeting an ANOVA q-value ≤ 0.01. Pairwise t-test analysis, using a fold difference (FD) of ≥ 1.5 and a p-value of ≤ 0.05 as cut off reveled significant differences in the distribution of proteins between groups. Platelets of both SLE groups (small and normal sized) shared higher levels of forty proteins and twenty proteins were reduced, compared to HC. Cytoskeletal functions were overrepresentation in the group of reduced proteins, while proteins with higher levels in platelets from SLE patients included proteins associated with complement and autoantibody targets such as Beta-2-glycoprotein 1, Annexin A5, and Prothrombin. Platelets from SLE patients also shared an abundance in immunoglobulin proteins, with even greater accumulation in the normal sized platelets. SLE platelet heavy constant alpha 1 (r -0.85, p=0.003), heavy constant mu (r -0.64, p=0.05) and heavy constant gamma 3 (r -0.80, p=0.008) was inversely correlated with complement C4 in serum and heavy constant gamma 2 (r -0.648, p=0.049) with complement C3. Conclusion This study revealed an accumulation of complement proteins, immunoglobulins and known autoantigens in platelets from SLE patients compared to HC. The signature was largely independent of platelet size, but the enrichment of proteins involved in SLE pathogenesis indicates that the composition is influenced by SLE disease mechanisms. This was supported by the inverse correlation between platelet immunoglobulin and serum levels of complement protein C3 and C4. Platelets are known to interact with complement and express the low-affinity immunoglobulin gamma Fc region receptor IIA (CD32), suggesting a role in the clearance of immune complexes [3]. Future studies will have to determine if platelets play a role in the turnover of complement and immune complexes and the potential role of platelets as a source of autoantigens. References [1]Linge, P., et al., The non-haemostatic role of platelets in systemic lupus erythematosus. Nat Rev Rheumatol, 2018. 14 (4): p. 195-213. [2]Lood, C., et al., Decreased platelet size is associated with platelet activation and anti-phospholipid syndrome in systemic lupus erythematosus. Rheumatology (Oxford), 2017. 56 (3): p. 408-416. [3]Huang, Z.Y., et al., Human platelet FcgammaRIIA and phagocytes in immune-complex clearance. Mol Immunol, 2011. 48 (4): p. 691-6. Disclosure of Interests Petrus Linge: None declared, Andreas Jern: None declared, Helena Tydén: None declared, Birgitta Gullstrand: None declared, Hong Yan: None declared, Charlotte Welinder: None declared, Robin Kahn: None declared, Andreas Jonsen Consultant of: Astra Zeneca and glaxosmithkline, John Semple: None declared, Anders Bengtsson: None declared.